This invention is directed to plastic containers, and more particularly, to plastic containers and a process for forming the containers having molded feet, wherein the thickness of the wall of the container forming the molded feet is preferably thicker than the cylindrical sidewalls of the container.
Footed plastic containers have been developed for giving containers additional stability so that they may stand erect. The footed containers are generally characterized by a base having several downwardly extending protrusions, in the form of feet, upon which the bottom may rest on a surface in the upright position. While this construction provides stability, it is not favorable for several reasons.
During the manufacture of the footed containers, preforms must be stretched and blown in a blow molding operation. The material forming the wall of the container becomes thinner as it is expanded from its original position as the preform wall. As the wall thins during expansion via blowing, it also cools and as it cools, it becomes increasingly difficult to deform, thereby requiring high pressure for continuing the deforming of the preforms. Moreover, by the time feet are formed by blowing, which is typically after the main cylindrical body of the bottle has been blown into contact with the mold, the deforming wall may have a temperature much lower than best suited for orientation. Accordingly, the feet of the container may become overstretched, resulting in increased brittleness and the likelihood of stress cracking of the bottle in the footed area while in service. These shortfalls hold true for most containers having footed bottoms. However, the degree to which problems are experienced, depends on the specific geometry of the feet.
The origin of the aforementioned problems is in the specific design of the preforms which are used to form the footed containers. Typically, the preforms used are in the shape of a test tube having an annular cross section in the regions that are to form the body and base of the bottle, wherein the thickness of the annular cross-section is dependent on the axial location of the section. The portion of the preform that is adapted to be blown into the feet is also annular. As a result, the thickness of the container walls will tend to be greater than the thickness at the deepest region of the feet. This occurs because the thickness at the annular cross-sections of the preform for all sections to be molded are substantially the same yet the amount of expansion required in the area to be blown into the feet is greater. The following patents are directed to preforms and containers, some of which have feet and all of which suffer from the shortfalls described above.
U.S. Pat. No. 3,598,270 discloses one of the first footed containers and is entitled "Bottom End Structure for Plastic Containers". The patent discloses a blow molded plastic container having a tubular body terminating at a generally hemispherical outwardly convexed bottom wall with a lowermost polar portion. The bottom wall is interrupted by a plurality of downwardly projecting wall portions defining hollow legs with adjacent legs being spaced from each other by meridian wall portions of the bottom wall. At least two of the meridian wall portions are disposed in generally diametrically opposed relationship and the two meridian walls have a common parison weld along an arc of a radius equal to that of the hemispherical bottom wall. The hollow legs terminate in feet having radially inwardly directed apex wall portions. The apex wall portions and at least two meridian wall portions merge at the polar portion. The preform used for forming the container described is essentially as described above, wherein the thickness of the walls of the preform are substantially uniform regardless of the portion of the container to which the walls are to be blown. As discussed above, this causes the resulting footed container to have substantially overstretched feet.
U.S. Pat. No. 4,978,015 to Walker discloses another footed container for pressurized fluids. It has a generally cylindrical sidewall portion and a bottom structure which comprises a central portion, a plurality of ribs extending downwardly from the sidewall portion to the central portion, and a plurality of feet extending below the central portion from the sidewall portion. The ribs are defined by an upper curvilinear surface and, in cross-section, are substantially U-shaped having a relatively tight radius. The upper surfaces of the ribs lie on a generally hemispherical curvature in the interior of the container. Each foot is positioned between two ribs and has a pair of ribs defining end walls connected to and continuous with the ribs on each side of the curvilinear outer wall. The curvilinear outer wall is connected to and continuous with the sidewall portion. A generally horizontal base surface is joined to the outer wall and a generally vertical first inner surface forms a lip extending upwardly from the base surface. A second inner surface extends from the lip of the central portion. As can be seen from the figures, the feet portions of the Walker container have substantially the same thickness as the cylindrical sidewall portions. Accordingly, the Walker container suffers from the same shortfalls discussed above.
U.S. Pat. No. 4,885,197 to Strassheimer discloses a plastic preform for forming blow molded non-footed plastic bottles. The plastic preform includes a tubular body portion having an inside wall face and a bottom structure, the tubular body portion adjacent the bottom structure and extending into the bottom structure has a plurality of flat faces with terminal portions thereof. Scalloped-shaped segments are provided for connecting the terminal portions of each flat face so as to initiate deformation of the preform at the scalloped-shaped segments, prior to deformation of the flat faces. As shown in FIGS. 4 and 6 of this patent, the preform is used for forming a container having a continuous peripheral seating ring with alternating thicker and thinner portions upon which the bottle rests which surrounds a generally concave central portion. In order to secure stability in such a design, the thickness of the base is made greater than in footed bottles if stability is to be obtained and wobbling avoided.
There exists, therefore, a need for a footed container and a process for forming the same wherein a uniform wall thickness of the base is obtained by forming the feet from preform walls having greater thickness than the preform walls used to form the rest of the base, i.e., by varying the corresponding cross sectional thickness of the preform.